Wave interference silencing system

ABSTRACT

A wave interference silencing system for predictably attenuating sound waves in the frequency noise spectrum of the exhaust gases from an internal combustion engine wherein a plurality of standardized serially connected attenuating sections includes a pair of discrete flow paths of differing acoustical lengths that are energy and flow compensated to establish actual sound propagational velocities so that the pressure waves of the individual paths are in phase opposition at the exit end of the transited attenuating section so as to destructively interfere within a low-impedance coupling volume adjacent thereto. The coupling volume serves to equalize the vibrational energy between the separate flow paths and effect a flow stabilization at the entrance end of a succeeding attenuating section.

United States Patent Inventor Donald R. Whitney Birmingham, Mich.

Appl. No. 39,705

Filed May 22, 1970 Patented May 25, 1971 Assignee General Motors Corporation Detroit, Mich.

WAVE INTERFERENCE SILENCING SYSTEM 4 Claims, 14 Drawing Figs.

Primary Examiner-Robert S. Ward, Jr. Attorneys-J. L. Carpenter, E. J. Biskup and Peter D. Sachtjen ABSTRACT: A wave interference silencing system for predictably attenuating sound waves in the frequency noise spectrum of the exhaust gases from an internal combustion engine wherein a plurality of standardized serially connected attenuating sections includes a pair of discrete flow paths of differing acoustical lengths that are energy and flow compensated to establish actual sound propagational velocities so that the pressure waves of the individual paths are in phase opposition at the exit end of the transited attenuating section so as to destructively interfere within a low-impedance coupling volume adjacent thereto. The coupling volume serves to equalize the vibrational energy between the separate flow paths and effect a flow stabilization at the entrance end of a succeeding attenuating section.

WAVE INTERFERENCE SILENCING SYSTEM This invention relates to exhaust silencing systems for motor vehicles and, in particular, to wave interference silencing systems incorporatinga plurality of serially connected attenuating sections.

The periodic noises produced by an operating motor vehicle are generally comprised of engine-related, low frequency noises ranging around 200 Hz. and high frequency noises ranging around 700 to 2,000 Hz. The 'low frequency noises emanate from the exhaust tailpipe and are primarily a function of engine-operating conditions, the exhaust system .pipe lengths, and the sound velocity in and of the exhaust gas. These noises are conventionally suppressed by nonflow resonant chambers utilizing a spring mass damping concept to silence a particular undesirable frequency. These nonflow chambers, depending on the acoustical nature of the emitted noises and the space availability beneath the vehicle chassis, may be incorporated into the systemas separate components or as parts of the vehicle muffler. The high frequency noises are generally a function of the combustion process, the engine speed,.restrictions in the gas flow stream, and the higher harmonies of exhaust pipe lengths. These frequencies are normally suppressed by flow sections incorporating baffles or labyrinths within'the engine muffler. Of course, other wellknown configurations can also be used to supplement the above components in either frequency range.

Regardless of the combination of acoustical elements adopted for these systems, the design for a given system requires a trial-and-error selection and matching of individual attentuating devices of varying sizes and characteristics in order to produce an acceptable silencing for a single-engine size. Due to the unpredictability of the interaction between the silencing components, a completely independent design process is necessary to achieve the desired attenuation for differing engine sizes.

The present invention contemplates providing attenuating sections of a single sectional design that can be serially incorporated into an exhaust system to predictably produce predetermined attenuation in the exhaust gas noise spectrum. Generally, these beneficial results are accomplished by a novel application of the principles of acoustical wave interference. The present arrangement takes the form of a pair of parallel flow paths having differing actual acoustical lengths so as to appreciably place the sound waves'in phase opposition at the ends of an individual section. In this manner, the gas streams travel on separate pathsand the pressure waves are substantially one-half period or 180 out of phase at the exit end of the transited attenuating section. Therefore, insofar as a given section is concerned, the output pressure due to-the mixing of the separate pressure waves produces a complete pressure stabilization and a band of attenuation on either side of the design frequency. Of the wave interference attenuating devices which have been heretofore proposedin the prior art, none have provided devices which adequately compensate for the variety of noises and conditions presented by motor vehicle exhaust lines. For instance, most devices have merely arbitrarily established mechanical length ratios which evidence little appreciation for the environmental conditions under which the device is to be operated. Without adequate regard for operating temperatures, flow processes, and flow energies of the individual flow paths, the attenuation thus provided is as unpredictable in frequency response as conventional systems. Moreover, individual devices of this type without adequate compensation for environment are not consonant in plural usage to form a coherent exhaust silencing system.

The present invention, on the other hand, utilizes a plurality of wave interference attenuating sections having a standardized cross-sectional design varying only in section lengths which are individually and collectively designed to produce a predictable noise suppression. Such sections also provide an exhaust system wherein the several sections are individually flow compensated both with regard to temperature and flow velocity so as to produce discrete flow paths having a maximum phase angle difference for a given system length while, at the same time, maintaining a minimum pressure loss due to flow velocity for a given pipe diameter. This standardized section can be appropriately sized insofar as section length is concemed to provide a predictable attenuation response at select frequency bands within the noise spectrum. Inasmuch as the standardized sections are independent of engine and environmental conditions, the entire system can be analytically designed by subsequent consideration of a single parameter, namely, the section length necessary to individually and collectively produce the requisite attenuation over the entire frequency spectrum.

These and other features will be apparent to one skilled in the art upon reading the following detailed description,-

reference being made to the accompanying drawings in which:

FIG. I is a plan view of a motor vehicle incorporating a wave interference silencing system made in accordance with the present invention;

FIG. -2 is a fragmentary cross-sectional view of an attenuating section; I FIG. 3 is a fragmentary cross-sectional view of an acoustical equalization conduit;

FIG. 4 is an exploded view of an attenuating section;

FIG. 5 is an enlarged view of a portion of an attenuating section;

FIGS. 6a through 60 are schematic views illustrating the effect of environmental flow conditions on sound propogational velocity and wave interference attenuation;

FIGS. 7a through 7e are graphs of predominant attenuating bands versus frequency for the individual attenuating sections; and

FIG. 7f isa composite graph of attenuation versus frequency forthe wave interference silencing system shown in FIG. 1.

Referring to'FIG. 1, there is shown a frame 10 for a motor vehicle on which there is mounted an internal combustion engine 12 having exhaust manifolds 14. For the V'-type engine illustrated, exhaust gases are discharged from the exhaust manifolds 14 to an exhaust line 16 including a crossover pipe 18. The exhaust line 16 includes a wave interference silencing system 19 comprising a: plurality of serially connected attenuating sections 20a, 20b, 20c, 20d, and 20e which are'axially separated byacoustical equalization conduits 22a, 22b, 22c, and 22d. A tailpipe extension 24 is telescoped over the rearward end of the attenuating section 20e .and constitutes the exhaust end of the system. Those skilled in the art will appreciate that the above components from the exhaust manifolds 14V to the tailpipe extension 24 form an acoustical line through which heated exhaust gases and their resultant pressure waves flowas products of combustion from the engine cylinders.

During operation, the engine 12 generates pressure pulses and sound waves in the exhaust line having plural frequencies which are a function of engine speed. For a given engine, certain exhaust frequencies will predominate in the noise spectrum of the exhaust gases. The present invention predictably attenuates the entire noise spectrum in the manner hereinafter described.

Referringto FIG. 2, a representative attenuating section 20' includes an inner conduit 30 and an outer sleeve 32 structurally connected by ahelical baffle 34. While the cross-sectional configuration of the attenuating section '20 .may take various forms and still accommodate the principles disclosed herein, the present embodiment utilizes a cylindrical tube for the inner conduit 30, a helically wound strip for the baffle 34,

and a helically wrapped sheet for the outer sleeve '32. The baffle 34 establishes a helical angle 0 and an axial pitch PIThe inner cylindrical edge of the baffle 34 is seam welded at .36 to .the outer surface of the inner conduit 30. The mating edges 40 sleeve 32 has an inner diameter D and an outer diameter D,,. The inner diameter D, forms an inner flow path 50 handling a gas flow Q, having an actual propagational sound velocity V,, a prevailing temperature T,, a cross-sectional area A,, and a length L,. The outer flow path 52 defined between the baffle 34 and the sleeve 32 and the inner conduit 30 handles a gas flow Q, having an actual propagational sound velocity V,,, a prevailing temperature T,,, a cross-sectional area A and a length L,,.

A representative acoustical equalization conduit 22', as shown in FIG. 3, is in the form of a continuous pipe having diagrammatically enlarged ends 60 which are telescopically received over the end sections 62 of adjacent attenuating sections 20'. Each conduit 22 has a length L, and an inner diameter D, and forms a flow stabilization and equalization volume 64 handling a gas flow Q,.

The method employed for attenuating sound waves in the present invention employs a main duct or primary flow path and a reentrant duct or parallel flow path. The flow paths have an acoustical length ratio, as hereinafter described, to establish an appreciable phase angle difference between the sound waves exiting at the equalization conduit. The method provides maximum attenuation or predominant attenuation bands when the phase angle is 180 or at odd-numbered multiples thereof, i.e., 540, 900. Supplemental bands of attenua tion are provided according to accepted theory when the sum of the respective flow path lengths, L, and L,,, are related by the formula L,+L,,=KA21m wherein n is an integer and K is a constant dependent upon frequency. With this arrangement, a pressure peak traveling through the reentrant duct will meet with a rarefaction at the exit end of the attenuating section' thereby causing destructive acoustical interference. Theoretically, if the sound waves and energies are equal in both ducts, the attenuation at this frequency will be infinite. Thus, as schematically shown in FIG. 60, separate sound waves will travel a reentrant path A and a main path B between an entrance end 100 and an exit end 102. When the flow velocities and energies are compensated and equalized, the sound waves will exhaust at the succeeding equalization volume so as to completely destructively interfere. In other words, the pressure waves at the exit end 102 will be in phase opposition and have equal and opposite peak amplitudes P and P thereby causing a complete equalization of pressure to produce a resultant ambient pressure amplitude. For the conventional attenuation formula A 20 log A B wherein A is the attenuation in decibels;

P,,-P is the resulting acoustical pressure at the exit end;

and

P is the acoustical pressure at the entrance end; the idealized conditions will produce infinite attenuation. However, the actual attenuating capabilities of any device are dependent on many variables such as system impedance, multiple internal reflections, and manufacturing inaccuracies. Therefore, the reasonable attenuation is generally established at a maximum value which for the purposes of subsequent description will be 20 db. or a pressure ratio of to l.

In the present. invention, the acoustical lengths of the separate flow paths 52 and 50 are selected to have an actual acoustical length ratio J which will establish the aforementioned phase angle relationship at the prevailing environmental conditions.

The ratio of flow path lengths to attenuate a given frequency has been found to require compensation for prevailing operating conditions such as temperature and gas velocity. Also, the inner and outer flow paths must be suitably related to actual operating conditions in order to balance the energy levels therewithin and maximize the wave interference attenuating capabilities of each section. Additionally, proper account must be taken of other factors such as flow resistance or pressure drop within the conduits and their effect on the length ratios between the inner and outer flow paths as well as the acoustical energy flowing within the separate paths.

Referring to FIG. 6a where the system is adequately temperature and flow compensated, the gas streams will be divided into two flow paths which have a relationship as follows:

a la i o ln other words, the pressure wave B flowing down the inner flow path will have an actual sound propagational velocity V, traveling through the length L, and a pressure peak P, exiting at the exit end 102 after a transit time t,. In the outer flow path, the pressure wave A will have an actual sound propagational velocity V, traveling through the outer length L, with a pressure P exiting the exit end 102 in transit time t Where the flow conditions satisfy the above-noted criteria, the pressure wave of the outer flow path will exit at the end 102 at a pressure amplitude P, while the sound wave of the inner flow path will exit at a pressure amplitude P,. The pressure waves, in this case, will be of equal magnitude and in phase opposition such that complete destructive acoustical interference will occur. The compensated system, under these conditions, will theoretically achieve infinite attenuation.

Where the inner and outer flow paths are not flow compensated, a time lag At will exist between the exiting of the inner 7 sound wave B and the outer sound wave A. In such instances,

the peak amplitudes of the sound waves will experience a phase shift A0 such that the terminal pressure amplitude P of the outer sound wave A will no longer be in exact phase opposition to the pressure amplitude P, of the inner sound wave B. In other words, only partial cancellation of the pressures occur and the resultant pressure at the exit of this attenuating section will be some finite value which, accordingly, reduces the attenuation value.

In instances where an inadequate mixing volume precedes an attenuating section, the sound waves, A" and B", as shown in FIG. 6c, traveling through the respective flow paths will have differing initial pressure amplitudes. Under these circumstances, the peak pressure amplitude P," from the outer flow path, in addition to having a lesser maximum amplitude than the inner flow path, will also have an exit pressure P,,' below its maximum amplitude. Additionally, inlet flow conditions may cause the inner sound wave B" to lag behind the outer sound wave A" by a time increment At or by a phase shift A0 according to the formula These aforementioned conditions also result in a decrease of attentuating efficiency and represent a significant reduction in performance as compared to the compensated system represented in FIG. 6a.

The design of a given exhaust line will be preliminarily controlled by certain vehicle and engine characteristics such as engine size, vehicle length, and vehicle clearance. More particularly, the engine size and speed will determine the maximum gas flow which the exhaust system must handle. The vehicle length, to a large extent, will determine the maximum length of the exhaust system subject to bending and other rerouting procedures. The available space beneath the vehicle body will determine the maximum size of the conduits employed in the exhaust system. Inasmuch as many factors influence the varying spectrum of noises produced by an engine, certain initial conditions must be tentatively prescribed on the basis of prior experience in the art such that remaining variables can thereafter be systematically ascertained to produce acceptable attenuation levels in the system.

Accordingly, the exhaust line length should be roughly established at a value which-is substantially equal to the length of the vehicle. Such a figure of course, includes sufficient length to incorporate the required attenuating sections 20 and equalization conduits 22. Secondly, the maximum outer diameter of the sleeve 32 is limited to a value which can be fitted to the available clearance with the vehicle body. Thirdly, the cross-sectional flow area of the sleeve should adequately handle the maximum gas flow from the engine 12. Fourthly, the inner conduit 30 is selected at a commercially available pipe size to thereby further enhance the economics of the present system.

For the system thus far described, the aforementioned actual sound propagational velocities, V and V,,, will exist in the inner flow path 50 and the outer flow path 52, respectively. These actual sound propagational velocities are comprised of sound velocity at the prevailing temperature C and the gas flow velocity V, in the flow path. More particularly, the functional relationship of the velocity of sound C, as related to temperature can be expressed as follows:

T, is the average temperature in the flow path 50.

At the prevailing temperatures in the exhaust line of around l,l00F. and a temperature difference of around F. between the flow paths, the propagational velocity will vary approximately 12 fps. Unless such change is properly recognized in the system design, the decrease in attenuation occurs as outlined with respect to FIGS. 6b and 6c.

The gas flow velocity V in the respective flow paths will be a function of the mass gas flow, the size and characteristics of the flow path including the cross-sectional area and the prevailing operating temperature. For the inner flow path this relationship can generally be empirically set forth asfollows:

wherein H is the pressure drop per foot of section length,

in. hg./ft.

For a quantity Q, flowing through the inner path, a gas flow velocity V will exist as related by the following formula:

The outer path 52 will have a flow area which is the equivalent of a single pipe having an inner diameter 5;, represented as follows:

wherein W is the width of the mean diameter helix, in.; and

Y is the height of the helix or D,,D,,/2, in.

With the equivalent diameter thus calculated, the mass flow through the outer flow path Q can be represented as follows:

Using a preliminary value for an initial helix angle, 0,, necessary to produce the uncompensated length ratio 1,, the helical pitch P for the baffle will be prescribed according to the formula:

P b+ d) 2 tan (0 )+T cos (0 wherein T is the thickness of the baffle, in.

This calculated pitch P will then establish an outer wrap helix angle 0, as follows:

The acoustical characteristics of the sound waves in the individual sound paths are a function of the actual gas flow velocity and the temperature compensated gas flow. Thus, the lengthratio J between the flow paths must be approximately revised. Generally, the aforementioned differences can be accounted for as follows;

This revised length ratio will also provide secondary representations for the helix angle 6 and the pitch P as follows:

1 0 Atl'l. and (D D b+ a P 3.14- tan Additionally,

n b d T Thus, with two representations for the helix angle Band an intermediate representation for the pitch P, by iterative computation, the design helix angle can be calculated within prescribed. limits with the helical pitch P being appropriately revised in accordance therewith. As a result of the aforementioned computations, a basic attenuation section will be established which is standardized in regard to the structural characteristics of the flow paths. Moreover, the basic section produces an acoustical wave interference due to phase opposition at the end thereof which is accurately compensated for prevailing flow and temperature conditions.

On the basis of a standard attenuating section, the total system for silencing objectionable sound noises can be established for a given motor vehicle engine. The permissible length for the exhaust line of a vehicle with such an engine will be roughly established by the length between the exhaust manifolds 14' and the end of the vehicle body together with mechanical bends necessary to route the exhaust line around the vehicle chassis. Moreover, each engine will normally produce objectionable noises at predominate frequencies. The most objectionable of these frequencies forms the basis for establishing the length of a fundamental attenuating section which will influence selected lengths of the remaining sections.

In the preferred embodiment, the longest section is selected as the fundamental attenuating section and has an acoustical length which attenuates a fundamental frequency at the low end of the noise spectrum and, of course, multiple frequencies thereabove. The length L, of this section is determined by the followingformula:

V, is the propagational sound velocity of gas in the outer flow path;

1,, is the actual acoustical length ratio of the outer flow path to the inner flow path; and

F 3 is the frequency to be attenuated.

By way of example, the preferred embodiment in the present invention uses an internal combustion engine having a full throttle actual sound velocity of 2,000 f.p.s., a length ratio of 2.02, and a base attenuating frequency of 250 Hz. Accordingly, the length of the base attenuating section is 3.96 feet.

The attenuation provided by such an attenuating section is dependent on the mass gas flow in the inner and outer flow paths as related to total gas flow and the permissible pressure drop for the exhaust line. Thus, given the specified maximum gas flow Q and permissible pressure drop H, for the entire system, the aforementioned relationships for Q V Q and V are iteratively'computed to obtain the values for the unknown quantities.

The attenuation for a given section is a function of the input energy to the section in comparison with exit energy and, accordingly, requires due consideration for the prevailing conditions in their respective flow paths. The flow energy in the inner flow path E, is a function of the flow path area and sound and gas flow velocity as related in the following manner:

E K[(V +CAi) -0.78 -(D Similarly, the flow energy in the outer flow path E, is

o l( m+ As a function of frequency F,, the attenuating section length L and the acoustical length ratio 1,, the angular difference of path lengths relative to the frequency at velocity of propagation G can be expressed as follows:

The output energy IE at the end of a given attenuating section is a function of the G factor, the attenuation factor A, and the attenuating section length L,. This relationship can be expressed as follows:

R log Referring to FIGS. 7a through 7e, the attenuation versus frequency responses for the individual attenuating sections 20a through 20e are illustrated for the predominant attenuation bands. In each of these sections, the arbitrary maximum attenuation cutoff has been set at 20 db. As previously mentioned, the attenuating section 20e has an output curve 120e selected to attenuate a certain low-level noise frequency of around 270 Hz. On either side of the design fundamental frequency, the attenuation decreases in the manner predicated in the formula outlined above. However, for longer length sections, secondary attenuation bands will be provided at higher frequencies corresponding to the odd integer multiples of the fundamental frequency. Thus, the base section 20a will have a secondary attenuating frequency response 120e at about 810 l-lz., a third response 120e" at about 1,340 112., and a fourth response l20e at about 1,890 Hz. Each response has a maximum amplitude bandwidth of around 100 Hz.

With the attenuating capabilities of the base section thus established, the noise spectrum of the engine is reanalyzed and the next objectionable frequency dealt with by selection of appropriate attenuating lengths. For example, the attenuating section 20b is selected to attenuate a low frequency noise 12011 of around 420 c.p.s. Such an attenuating section will additiorially contribute to overall attenuation at the supplemental attenuating bands ranging around 1,260 112., 120d, and 2,100 I-Iz., 120d", with each response having a bandwidth of about 160 Hz.

The attenuating response for the exhaust system is then once again reexamined and an intermediate attenuating length selected to provide a noise suppression in the middle frequency range. The third attenuating section 200 has a fundamental frequency, 1200, of about 625 Hz., a secondary response, 1200', of 1,875 Hz., and a bandwidth of 310 Hz., The fourth section 20d is used to provide broad-band attenuation in the middle frequency range and has a first attenuating response, 120b, of about 990 I-Iz., and a bandwidth of about 550 Hz. The fifth attenuating section 202 is used for extremely broad-band, high frequency attenuation and has a fundamental attenuating frequency 120a of about 1,840 Hz.

The resulting attenuation of the exhaust line is shown in FIG. 7f and represents the algebraic summation of the attenuation provided by the individual sections. In the event that the attenuation provided at a particular frequency is greater than necessary or insufficient to meet the desired sound reduction, the individual section lengths can be appropriately revised to provide greater or lesser attenuation response for these portions of the spectrum. Moreover, additional sections can be incorporated within the overall length restriction for the exhaust line. Generally, however, the frequency spectrum,

to the extent possible, should attenuate with at least the arbitrary maximum factor A after the low frequency band. Accordingly, in the preferred embodiment, the section lengths are balanced such that the peak amplitude cutoffs for the given attenuating peaks are generally overlapping.

Regardless the sizing of the mixing sections, as previously noted, the mixing volume 64 of the equalization conduits 22 should provide for total mixing of the sound waves which have separately proceeded down the inner and outer flow paths. Moreover, they should provide for pressure and gas flow stabilization in order to minimize differences in vibrational pressures and velocities at the entrance of a subsequent section. While the diameter for the mixing section may be roughly the same as the inner diameter of the outer flow path, variances therein might be desirable to enable routing of the exhaust line around the chassis. Therefore, I have found that minimum permissible flow path is of greatest importance and is generally represented as the equivalent single-pipe diameter for handling combined gas flowof the inner and outer flow paths. More specifically, the inner diameter D, can be empirically represented as follows:

Q im mixing section should be about twice the diameter of S to achieve flow and pressure stabilization mentioned above.

An exhaust line having attenuating capabilities reported in FIGS. 6 through 1 1 has been successfully built and operated in accordance with the following dimensions:

Section length, feet:

Average temperature degree from outer path, FD, F 1,

Average temperature degree from inner path, Fi, F 1,120

Pressure drop per foot, H, in. hg./ft 1. 68

Actual acoustical length ratio, J a, ieet 2. 02

Helix angle 9, radians 511 Total gas flow:

a, inches. 1. 43

Db, inches. 1. 60

D n, inches 1. 93

Dd, inches 2.36

D Inches 2. 60

Although only one form at thisinvention has been shown and described, other forms will be readily apparent to those skilled in the art. Therefore, it is not intended to limit the scope of this invention by the embodiment selected for the purpose of this disclosure, but only by the claims which follow.

What I claim is:

l. A silencing system for attenuating exhaust gas sound waves from an internal combustion engine, comprising: a plurality of serially connected attenuating members having low acoustical impedance volumes fluidly coupled thereto; and a pair of discrete flow paths in each of said attenuating members that are individually temperature and flow compensated for the sound waves transiting therewithin so as to establish actual sound propagational velocities for the latter that will cause destructive interference in said volume at the exit end of the transited attenuating member thereby suppressing objectionable noises at predetermined frequencies, each attenuating member further including means for compensating the mass flow through the separate flow paths to equalize vibrational energy arriving at said volumes and thereby minimize vibrational pressures at the entrance end of a succeeding attenuating member.

2. A wave interference exhaust system for attenuating sound waves in the frequency spectrum produced in the exhaust gases of an internal combustion engine, comprising: an exhaust line including a plurality of attenuating sections acoustically coupled in series; conduit means of a low acoustical impedance fluidly succeeding each attenuating section; and a pair of alternate gas flow paths in each attenuating sectionwhich handle substantially equal vibrational energy and mass gas flow, and produce destructive interference in the succeeding conduit means, the ratio of the flow path lengths for the individual sections being effective to attenuate a predetermined frequency band in said spectrum, and with the remaining sections, cumulatively reduce objectionable vibrational energy to an acceptable level.

3. A wave interference silencing system for predictably attenuating sound waves at select frequency bands in the noise spectrum of the exhaust gases from an internal combustion engine, comprising: a plurality of attenuating sections acoustipaths in each attenuating section, said flow paths having differing acoustical lengths and being temperature and flowcompensated to establish actual sound propagational velocities so that pressure waves transiting therewithin are in appreciable phase opposition at the exit of the transited paths so as to destructively interfere within said equalizing conduits, said sections being standardized in sectional design and being of individual lengths which will provide predictable attenuation responses at said select frequency bands whereby subsequent consideration of a single parameter permits a system design of predictable attenuating response.

4. A method of predictably attenuating sound waves in the exhaust gases of an internal combustion engine, comprising the steps of:

. separating the exhaust gases into a pair of flow paths;

2. sizing the flow paths to handle equal vibrational energies;

3. dividing the flow paths into discrete sections;

4. establishing a length ratio between the flow paths in the individual sections which is flow and temperature compensated to place the sound waves in phase opposition;

5. selecting a discriminate length for the compensated sections to produce predictable attenuation responses at a select frequency band of the noise spectrum of the exhaust gases; and

6. combining the flow paths at low impedance volumes succeeding each section to cause the pressure waves transiting the individual flow paths to'destructively interfere and cumulatively produce an attenuated ambient acoustical pressure for the frequency noise spectrum of the engine. 

1. A silencing system for attenuating exhaust gas sound waves from an internal combustion engine, comprising: a plurality of serially connected attenuating members having low acoustical impedance volumes fluidly coupled thereto; and a pair of discrete flow paths in each of said attenuating members that are individually temperature and flow compensated for the sound waves transiting therewithin so as to establish actual sound propagational velocities for the latter that will cause destructive interference in said volume at the exit end of the transited attenuating member thereby suppressing objectionable noises at predetermined frequencies, each attenuating member further including means for compensating the mass flow through the separate flow paths to equalize vibrational energy arriving at said volumes and thereby minimize vibrational pressures at the entrance end of a succeeding attenuating member.
 2. A wave interference exhaust system for attenuating sound waves in the frequency spectrum produced in the exhaust gases of an internal combustion engine, comprising: an exhaust line including a plurality of attenuating sections acoustically coupled in series; conduit means of a low acoustical impedance fluidly succeeding each attenuating section; and a pair of alternate gas flow paths in each attenuating section which handle substantially equal vibrational energy and mass gas flow, and produce destructive interference in the succeeding conduit means, the ratio of the flow path lengths for the individual sections being effective to attenuate a predetermined frequency band in said spectrum, and with the remaining sections, cumulatively reduce objectionable vibrational energy to an acceptable level.
 2. sizing the flow paths to handle equal vibrational energies;
 3. dividing the flow paths into discrete sections;
 3. A wave interference silencing system for predictably attenuating sound waves at select frequency bands in the noise spectrum of the exhaust gases from an internal combustion engine, comprising: a plurality of attenuating sections acoustically coupled in series; acoustical equalizing conduits fluidly succeeding each attenuating sections; a pair of discrete flow paths in each attenuating section, said flow paths having differing acoustical lengths and being temperature and flow compensated to establish actual sound propagational velocities so that pressure waves transiting therewithin are in appreciable phase opposition at the exit of the transited paths so as to destructively interfere within said equalizing conduits, said sections being standardized in sectional design and being of individual lengths which will provide predictable attenuation responses at said select frequency bands whereby subsequent consideration of a single parameter permits a system design of predictable attenuating response.
 4. A method of predictably attenuating sound waves in the exhaust gases of an internal combustion engine, comprising the steps of:
 4. establishing a length ratio between the flow paths iN the individual sections which is flow and temperature compensated to place the sound waves in phase opposition;
 5. selecting a discriminate length for the compensated sections to produce predictable attenuation responses at a select frequency band of the noise spectrum of the exhaust gases; and
 6. combining the flow paths at low impedance volumes succeeding each section to cause the pressure waves transiting the individual flow paths to destructively interfere and cumulatively produce an attenuated ambient acoustical pressure for the frequency noise spectrum of the engine. 